1. Technical Field
The invention relates to the recovery of hydrocarbons and more specifically to a process for selectively reducing the permeability of a subterranean hydrocarbon-bearing formation.
2. Background Information
During recovery of hydrocarbons from a heterogeneous subterranean hydrocarbon-bearing formation, materials are often placed in the formation to modify the permeability of the formation and improve hydrocarbon recovery therefrom. For example, U.S. Pat. No. 4,683,949 to Sydansk et al discloses a process which utilizes a crosslinked polymer gel to selectively reduce permeability in the formation. Selectivity in this context is the ability to reduce permeability in the high permeability regions of the formation without reducing permeability in the low permeability regions thereof.
Permeability-reducing polymer gels known in the art, such as those disclosed in Sydansk et al, have been shown to be inherently more resistant to penetration of the low permeability regions of the formation than the high permeability regions, which is a favorable selectivity characteristic. Nevertheless, it has been found that when such gels are injected into heterogeneous formations, they usually penetrate not only the high permeability regions, but to some extent the low permeability regions. This problem is particularly acute when the difference between the permeabilities of the two regions is relatively small.
U.S. Pat. No. 4,809,781 to Hoefner, like Sydansk et al, is a permeability reduction process which relies on the inherent selectivity of a permeability-reducing material in a heterogeneous formation. The process of Hoefner is initiated by injecting a selective permeability-reducing material through a well bore into a treatment zone. The selective material must be chosen such that it is capable of penetrating the high permeability regions of the zone, but not the low permeability regions. Consequently, a portion of the selective material enters the high permeability region while the remainder accumulates at the face of the low permeability region thereby plugging the face.
A nonselective permeability-reducing material is subsequently injected into the well bore following the selective material. The selective material plugging the face of the low permeability region prevents entry of the nonselective material therein. As a result, the nonselective material is diverted into the high permeability region where it beneficially fortifies the selective material already in place. Finally, a chemical agent is injected into the well bore to remove the selective material from the face of the low permeability region and restore fluid pathways thereto.
The process of Hoefner requires careful selection of materials and rigorous process control for successful operation. However, it is extremely difficult to choose a selective permeability-reducing material which will perform as desired. If the material is not sufficiently selective it will penetrate the low permeability region, in which case the low permeability region will suffer unacceptable permeability damage. It is possible to reverse permeability damage with a strong reversing agent, but the agent is also likely to attack the permeability-reducing material in the high permeability region, thereby destroying the effectiveness of the entire permeability reduction treatment.
Processes have been developed for selective placement of permeability-reducing materials in subterranean hydrocarbon-bearing formations, which do not rely on the inherently selective properties of the permeability-reducing materials, but rely on more intrusive means of placement. Mechanical zone isolation is one such means for selective placement of materials in a treatment zone. The zone isolation equipment is designed to mechanically direct a permeability-reducing material exclusively into a high permeability region. However, mechanical zone isolation is ineffective where radial pathways exist which enable the injected material to bypass the zone isolation equipment and enter the low permeability regions.
As such, a process is needed for selectively placing a permeability-reducing material in a high permeability region of a subterranean formation without a significant risk of substantially damaging low permeability regions in fluid communication therewith. Specifically, a process is needed for the selective placement of a permeability-reducing material in a subterranean hydrocarbon-bearing formation which is relatively insensitive to the selectivity of the permeability-reducing material. Furthermore, a selective placement process is needed which is simpler and more economical, yet more effective, than conventional mechanical zone isolation processes.